This invention relates to a thin film photoelectric converting device, and more particularly a contact type image sensor utilized as a manuscript reading device such as a facsimile telegraph.
In recent years, in a manuscript reading device such as a facsimile telegraph, various types of the contact type sensors have been developed in which photoelectric systems can be miniaturized so that they correspond to respective manuscripts for the purpose of miniaturizing and decreasing the cost of manufacturing of the device. These previous sensors can be classified into storage and continuous type according to their method of driving. According to the former type, electric charges generated when one line of such photoelectric converting elements as photodiodes is scanned with light are integrated or stored and read out so that there are such advantages that the signal ratio between dark and bright is high and that response speed to light is high. Present day sensors utilize a chalcogenide film or an amorphous silicon film. Sensors utilizing the latter type do not store electric charge but detect the strength of light reflected at the surface of a manuscript as the variation in the resistance of the sensor by utilizing its characteristic as a photo-conductor. Thus, as it utilizes the secondary current of light its light response is slow.
Presently used storage type image sensors are defective in that it requires a specific IC circuit having performances of a shift register and an analog switch array. With the continuous type image sensor, by providing a rectifying contact for one electrode, it is possible to drive in a matrix mode, which can not only reduce the number of switches but also makes it unnecessary to use the specific IC circuit, thus reducing the cost. The defects of the storage type image sensor can be obviated by constructing a portion corresponding to the IC as a thin film transistor fabricated on the same substrate. This method, however, is not yet practical for the following two reasons. First reason lies in that the mobility of charge carries in the thin film transistor is generally much lower than that of a transistor made of a single crystal whereby its response is slow and high speed switching is impossible. Another reason lies in that it is difficult to uniformly fabricate a thin film transistor having a large area of a high reproducibility, and as an integrated circuit. Where a relatively high operating speed thin film transistor array and a contact type image sensor array are formed on the same substrate the manufacturing process becomes extremely complicated.